The effect of pre-aeration on the purification processes in the long-term performance of a horizontal subsurface flow constructed wetland.

Different conditions (water level, oxygen supply) prevailing in both beds of the Kodijärve double-bed horizontal subsurface flow (HSSF) constructed wetland (CW) (Southern Estonia; constructed in 1996, total area 312.5 m(2), 40 pe) provide the opportunity to compare how different operational methods have altered the efficiency of the purification processes inside the HSSF CW. In summer 2002 a vertical subsurface flow (VSSF) CW (total area 37.4 m(2)) was added as the first stage of the system. Data from 18 sampling wells installed in Kodijärve HSSF CW from two periods is compared: 1st period -- January 2000-April 2002 (before the VSSF CW was built); 2nd period --October 2002-December 2004 (after the construction of the VSSF filter). The VSSF CW has remarkably improved aerobic conditions in both beds of the HSSF. Apart from total phosphorus concentrations in the right bed and nitrate nitrogen concentrations in the outflow of both beds, all of the water quality indicators (dissolved oxygen, total suspended solids, biological oxygen demand, ammonia nitrogen, nitrite nitrogen, total nitrogen and total iron) improved after the construction of the VSSF filter. Typically, purification processes in the HSSF CW were dependent on oxygen supply, which was partly influenced by the water level inside the filter beds.

Alternative filter media for phosphorous removal in a horizontal subsurface flow constructed wetland.

During the study period from 1997 to 2002 the purification efficiency of phosphorus in the horizontal subsurface flow (HSSF) constructed wetland (CW) in Kodijärve, has been quite high (63-95%). However, slowly increasing trend in outlet P concentrations and decreasing annual P removal rate are obviously the indicators that show possible saturation processes in filter media. To search for potential filter media with high phosphorus sorption capacity, sorption characteristics and particle size distribution of several local sands, gravels, glauconite-sandstone, LWA, and calcareous waste products from oil-shale industry were investigated. The average P sorption capacity for best materials (crashed ash block, oil, shale fly ash and the sediment from oil shale ash plateau) was higher than 96% and estimated design capacity was around 4-5 g P kg(-1). According to results, sediment from oil shale ash plateau was considered as perspective filter media for P retention. In Summer 2002 experimental sedimentation filter, filled with the sediment from oil shale ash plateau, was installed in the outlet from the Kodijärve HSSF CW. According to preliminary results the average P removal in the sedimentation filter was 52%.

The effects of aeration and the application of the k-C* model in a subsurface flow constructed wetland.

Since the horizontal subsurface flow constructed wetland in Kodijärve, Estonia had problems with aeration, a vertical subsurface flow (VSSF) constructed wetland (CW) was built for the purpose of enhancing aeration. This article gives an overview of how the vertical subsurface flow constructed wetland has affected the aeration capacity and purification efficiency of the constructed wetland system in a one-year period of operation. The aeration capacity of the VSSF CW averaged 63.6 g O2 m(-2) d(-1) during this one-year operational period, and aerobic processes occur more effectively in the system. An area of 0.5 m2 pe(-1) is most likely too small for sufficient aeration in cold climates, and the short water residence time in the VSSF CW also can hinder nitrification. Also, an analysis of the application of the k-C* model in the horizontal subsurface flow part of the Kodijärve hybrid constructed wetland is presented for BOD, total nitrogen, ammonium and total phosphorus. The k-C* model describes processes in the HSSF CW quite well. The k values are found to be smaller than reported in the literature, mainly because of the low hydraulic loading rates. Low k values in more oxygen dependent processes, such as BOD, N(tot), and NH4-N removal, also indicate insufficient oxygen supply in the system.